This invention relates in general to techniques for producing thin films, and in particular to a method of using pulsed laser to deposit high temperature superconducting thin films.
Ever since the recent discovery of high T.sub.c superconductor materials, considerable effort has been expended to provide practical methods for making such materials. At this time, the only forms with practical application are thin films, which may be used in certain instruments such as SQUIDS and bolometers. Another application contemplated is fabrication of general purpose superconductors in the form of thin films deposited on wires or tapes. However, present techniques for producing thin films are generally very costly and very difficult to reproduce.
Of the many techniques for producing thin films from high T.sub.c superconducting ceramics, pulse-laser deposition has provided excellent films with narrow resistance transition widths and high critical currents As currently practiced, the technique employs a series of very short (nanosecond duration) pulses, principally from excimer lasers to ablate the surface of bulk superconducting material and to deposit the material as a thin film onto a substrate. The peak power densities of the laser irradiation are in the range 10.sup.7 -10.sup.9 W/cm.sup.2, in which case laser-vapor interaction may occur and the blowoff may contain high-energy ions as well as neutral species.
One critical consideration in deposition of high-T.sub.c superconducting films is the requirement for congruent vaporization of the constituent atoms in the superconducting material such that the proper metal-atom ratios are preserved in the film. Maintaining the original material's stoichiometry during vaporization is essential for preserving the superconducting structure and ensures a sharp transition temperature and a high current density. It is generally believed that the combination of high power density and low deposition rate (a few Angstrom per pulse at most) per pulse obtainable from short-pulse (nanosecond duration) excimer lasers is necessary for achieving congruent vaporization. The reasoning is that such a short pulse will provide a shock mechanism for dislodging the constituent atoms rather than relying on equilibrium thermal processes, which may lead to non-congruent vaporization.
Although high quality films can be produced by short-pulsed lasers, practical industrial application is problematic. First, the narrow angular distribution in the vapor blowoff makes deposition of uniform thickness films over large substrate areas difficult. Typically the angular distribution of the emitted material is non-diffuse, with functional forms as peaked as cos.sup.8 .PHI. or even cos.sup.11 .PHI. (where .PHI. is the emission angle relative to the normal of the film surface). This amounts to a 25% reduction in thickness outside a cone of angle 10-15.degree.. Secondly, the throughput is small with a deposition rate of approximately 1 .ANG. per pulse. About 10.sup.3 pulses are required to deposit a film of 0.1 .mu.m thickness, which can take approximately 30 minutes. Thirdly, the initial laser cost and maintenance expenses are high.
With the low deposition rate for short-pulse lasers, any lower peak power density irradiation will be impractical. For example, nanosecond-wide pulses with a repetition rate of .about.1Hz and peak power density of 10.sup.5 -10.sup.6 W/cm.sup.2 simply is ineffective in producing sufficient vaporization to deposit a film in a reasonable period of time
Longer pulse lasers would provide faster deposition rates; however, it was generally believed that they will lead to non-congruent vaporization, and therefore the deposited films would not have the correct stoichiometry needed for superconductivity.